1. Field of the Invention
The present invention relates to spread spectrum communication systems using PN coding techniques and, more particularly, to provide dynamic calibration for power balancing imperative to high performance CDMA systems.
2. Prior Art
Spread spectrum (SS) systems, which may be CDMA systems, are well known in the art. SS systems can employ a transmission technique in which a pseudo-noise (PN) PN-code is used as a modulating waveform to spread the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiver the signal is de-spread using a synchronized replica of the PN-code.
There are, in general, two basic types of SS systems: direct sequence spread spectrum systems (DSSS) and frequency hop spread spectrum systems (FHSS).
The DSSS systems spread the signal over a bandwidth fRF±Rc, where fRF represents the center bandpass carrier frequency and Rc represents the PN-code maximum chip rate, which in turn is an integer multiple of the symbol rate Rs. Multiple access systems employ DSSS techniques when transmitting multiple channels over the same frequency bandwidth to multiple receivers, each receiver having its own designated PN-code. Although each receiver receives the entire frequency bandwidth only the signal with the receiver's matching PN-code will appear intelligible, the rest appears as noise that is easily filtered. These systems are well known in the art and will not be discussed further.
As noted, the DHSS system PN-code sequence spreads the data signal over the available bandwidth such that the carrier appears to be noise-like and random to a receiver not using the same PN-code.
In communication systems having a central base station or hub and multiple subscriber units or consumer premise equipment (CPE), e.g., mobile units, the base station receives and decodes signals transmitted by each of the mobile units. It will be appreciated that in a CDMA type system, the signals transmitted by the mobile units preferably arrive at the base station with similar power levels; otherwise, interference may result and/or the gain control circuitry of the base station may suppress signals with comparatively lower power levels.
In order to regulate the received signal power levels many communication systems employ an open loop power control scheme. In this scheme the forward (base station to subscriber) channel loss is estimated by the subscriber unit measuring the total received power and combining this measurement with certain nominal base station parameters to calculate the estimated channel loss. The subscriber unit then adjusts its transmission power to compensate for the estimated channel loss. In this manner, and with all the subscriber units within the system using the same process, the power level from each subscriber unit received at the base station can be made to be substantially alike. However, open loop power control schemes generally require that the CPE needs to be calibrated for the open loop power control algorithm to generate accurate power control. However, it is often undesirable to pre-calibrate the subscriber unit because of expense.
Other communication systems may use a closed loop power control algorithm whereby the base station directly measures the received power from the subscriber unit and issues power level control signals, generally in the form of power step commands, to the subscriber unit to bring the received power level in line with operating conditions. However, under many conditions the closed loop approach may not respond quickly enough to compensate for operational conditions such as fading, thereby resulting in corrupted data communications.
Therefore, it is desirable to provide a method and system whereby the power level of signals transmitted by subscriber units may be controlled in an efficient and effective manner to compensate for power loss due to transmission channel conditions.